Germanium, a group-IV semiconductor material, offers a wide range of established and potential technological applications of high relevance and impact. In optoelectronics, Ge has been used as a photodetector material for use in on-chip data distribution, thanks to its strong interband absorption at near-infrared optical communication wavelengths, and again to its direct compatibility with the Si microelectronics platform. Additional device applications within the emerging field of group-IV photonics, including lasers and solar cells, have also been investigated.